Locating items using radio location systems has been performed using many different systems. Global positioning systems are quite useful for determining the location of an item which is outdoors, or otherwise has a clear path for signals from GPS satellites. However, when items are indoors, or otherwise surrounded by other items which might reflect such signals, it is difficult to determine an accurate time of arrival of location signals transmitted or received by the items. This is referred to as a multipath problem because location signals may take multiple paths, and appear to arrive at one or more receivers at different times. RF transmitters are commonly used for such applications. They are also used on robots or other items which may be outfitted with various sensors, and move within a building or other structure. It becomes imperative to know within a small distance, where such items are located.
Methods for determining the position of an RF transmitter have been studied extensively. A common approach to these systems is to use time difference of arrival (TDOA) information. In these TDOA systems a number of receivers detect a signal that was transmitted. The receivers must determine when the signal was received. The accuracy required for the timing information depends on the precision needed in determining the transmitter's location. If a position accuracy of one meter is needed, then the accuracy of the timing information must be better than approximately 3 nanoseconds. This relationship is fundamental and is simply due to the speed of light, which is 30 cm per nanosecond. The timing information is then converted into time difference data by calculating the difference in receive times between pairs of receivers. Knowledge of the position of two receivers, and knowledge of the difference in time of arrival, determines a hyperboloid which represents the potential positions of the transmitter. More than two receivers are used to create multiple hyperboloids, whose intersection represent the actual position of the transmitter.
As these systems must typically determine the time of arrival of signals to a precision of approximately one nanosecond they have typically used wideband RF signals in order to allow the receivers to detect very fast rising edge pulses. U.S. Pat. No. 5,742,636 to Sanderford, Jr., uses a spread spectrum modulation to time stamp a received radio broadcast. Wideband approaches consume significant amounts of power, which is a precious commodity for small robot like items carrying sensors. Further, such signals are not directly compatible with the desire for narrowband RF signals that are used to carry data. There is a need for a location system that is accurate to fairly small distances to aid in locating items coupled to the source of RF signals. The system must also provide location information in real time to handle items that move. There is a further need for location systems that operate with low power requirements for the items being located. There is still a further need for such systems wherein the devices that detect the items are mobile.